Optical displacement meters are used for measuring the displacement, distance, surface shape, and the like for a measurement subject.
When a measurement subject is measured with optical displacement meters, the measurement subject is irradiated with measurement light, reflected light from the measurement subject is received by a light-receiving element, and an output signal from the light-receiving element is processed to calculate an expected numerical value. For example, based on the output signal of the light-receiving element, a change in the light-receiving position of the reflected light received by the light-receiving element can be detected, and the displacement of the measurement subject can be measured.
In optical displacement meters, the light-receiving sensitivity, that is, the relationship of the output signal with respect to the amount of received light, may be problematic.
In FIG. 5, for the characteristic SL that has a low light-receiving sensitivity, the change in the output signal V is small with respect to the change in the amount of received light U; that is, since the detection resolution is small, the optical displacement meter cannot detect minute changes.
In contrast, for the characteristic SH that has a high light-receiving sensitivity, the change in the output signal V is large with respect to the change in the amount of received light U, so that the detection resolution can be increased. However, since the maximum value Ux of the amount of received light corresponding to the maximum value Vx of the output signal becomes small, in a case where there is an excessive amount of received light that exceeds this value, the output signal V is saturated at the maximum value Vx, and proper detection cannot be performed.
As an example of a situation in which an excessive amount of received light is obtained, there are cases when a light-receiving element for receiving diffuse reflected light of a measurement subject receives specular reflection light. That is, to detect reflected light from any direction, the optical displacement meter is set to receive the diffuse reflected light of the measurement subject. However, in a case where the measurement subject has a smooth surface, the measurement light is reflected thereon, and is incident on the light-receiving element according to a specific angular condition. Light from specular reflection is significantly stronger than ordinary diffuse reflected light, so that the light-receiving element receives an amount of light several times greater than that of ordinary diffuse reflected light, and the output signal becomes saturated.
To avoid such saturation, optical displacement meters have been proposed in which the light-receiving characteristic of the light-receiving element is adjusted so as to have a high sensitivity in regions where the amount of received light is small (see region A1 and characteristic S1 in FIG. 6), and have a low sensitivity in regions where the amount of received light is large (see region A2 and characteristic S2 in FIG. 6), so that even in cases where an excessive amount of light is received, the output signal will fall within the range of the maximum value of the output signal (see Patent Document 1). Logarithmic curves, quadratic curves, or the like can suitably be used as light-receiving characteristics in which sensitivity is high in regions where the amount of received light is small, and the sensitivity is low in regions where the amount of received light is large.